Compounds having strong absorptions in a range of from 450 nm to 1,100 nm, especially those having maximum absorptions (λmax) between 480 and 620 nm are used as optical elements for the optical recording layers of optical recording media such as DVD±R and the like, and for optical filters for the image display units such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescence displays (ELD), cathode ray tube displays (CRT), fluorescent displays, field emission displays, and the like.
For example, one application of the optical element in the image display unit is a light absorber for the color filter. The image display unit displays color images by a combination of three primary colors, namely, red, blue, and green. However, light which displays color images contains light between green and red, for example, between 550 and 600 nm, which causes lowering of the display quality. Also, light between 750 and 1,100 nm is contained, which may cause a malfunction of an infrared remote control. Thus, there is used an optical filter containing a light absorbing compound (a light absorber) which selectively absorbs light of these unnecessary wavelengths.
Further, the optical filter is also desired to have a function of preventing reflection or glare of outside light such as fluorescent lamps and the like. In order to prevent reflection or glare, the optical filter needs to absorb light of wavelength between 480 and 500 nm, in addition to its function to selectively absorb the above-mentioned light of unnecessary wavelengths. The light in this region is close to the bright line needed to display images. Thus, in order not to affect the image quality, the light absorption curve by the light absorber is required to be especially steep, namely the half bandwidth at λmax is required to be small.
Furthermore, the light absorber is also required not to lose its function by light, heat, and the like.
As an optical filter comprising the light absorber, for example, the following Patent Document 1 discloses an optical filter which uses a dipyrromethene metal chelate compound having a maximum absorption wavelength between 440 and 510 nm, and Patent Document 2 discloses an optical filter which utilizes a porphyrin compound having a maximum absorption wavelength between 440 and 510 nm. However, the compounds used for these optical filters do not provide satisfactory performances in terms of absorption wavelength characteristics or compatibility with the solvents and binder resins. Thus, these optical filters did not show satisfactory performances between 480 and 500 nm.
Further, in the above-mentioned optical recording media, the wavelength of a semiconductor laser used for recording and reproduction is between 750 and 830 nm for CD-R and between 620 and 690 nm for DVD±R. However, in order to realize further increase in the recording capacity, an optical disc which uses short-wavelength laser light, for example, one which uses recording light of 380 to 420 nm in wavelength is under study.
In an optical recording medium for the short-wavelength recording light, various compounds are used to form the optical recording layer. For example, there are reported in Patent Document 3 an cyanine-based dye, in Patent Document 4 a metal complex of triazole compounds, and in Patent Document 5 a porphyrin compound. However, these compounds were not necessarily adequate as an optical recording material used for formation of an optical recording layer, in terms of absorption wavelength characteristics and light resistance.
Patent Document No. 1: Japanese Patent Laid-Open Publication No. 2003-57436
Patent Document No. 2: Japanese Patent Laid-Open Publication No. 2004-45887
Patent Document No. 3: Japanese Patent Laid-Open Publication No. 2001-301333
Patent Document No. 4: Japanese Patent Laid-Open Publication No. 2004-174838
Patent Document No. 5: Japanese Patent Laid-Open Publication No. 2005-59601